Simulators require a means of connecting the digital computer, which represents information by bit patterns in its memory, to external simulation equipment, which represents information by switch closures, indicator lights, control stick deflections, etc. The linkage between the computer and the external devices must handle two classes of data: (1) analog data, where the computer information is translated into a physical quantity, and (2) discrete data, where computer information is translated into on/off signals such as switch closures. The translation from analog and discrete data to the computer representation of information is also required.
Most simulators perform the conversion in hardware that is physically adjacent to the central computer, thus requiring hundreds of interconnection lines. Cable runs of fifty to one hundred feet are not uncommon, and the mass of wire presents a significant problem. Additionally, many of the cables carry analog signals which are more susceptable to noise than digital signals, thus dictating shielding of signal lines and a more elaborate grounding system than is required for digital signal transmission.
Placing the conversion equipment at the place where the signal is used, rather than at the digital computer avoids the cabling problems described, inasmuch as data is transmitted via a serial data transmission line consisting of only a few signal lines. However, a digital computer system can provide data one to two orders of magnitude faster than data can be transmitted over a serial line. Delay in the transmission of data adds directly to computational delays, resulting in an overall lag between student input to a simulator and the correct simulator response.